The present invention relates generally to induction heating and in particular to an induction heating apparatus of the type in which the duration of forward current trigger is varied in accordance with varying loads to assure commutation of backward current.
Induction heating for houshold and industrial applications is found to be attractive because of its capability to transfer energy at high effeiciency as compared with resistance heating. However, cooking operation involves the use of utensils of various sizes, which results in undesirable consequences of a failure in trigger operations since the switching devices employed are not given sufficient time to turn off prior to the subsequent trigger instant. The induction heating apparatus which have hitherto been proposed employ constant duration trigger or gating control pulses to trigger the switching devices to generate a forward current in a commutating circuit which in response thereto generates a backward commutating current. Since the backward current flows in opposite phase relation with the polarity of the input power source, the commutation of such current in the second half-cycle of each oscillation represents a negative power, which offers an advantage in that a certain amount of power is returned to the input side of the apparatus. Therefore, it is important to assure that commutation should occur at all times. However, when load changes occur during operation the period of each forward and backward current tends to vary so that the conventional constant duration gating control approach fails to assure commutation with the attendant loss of recoverable energy.